Menstrual Flow Calculator
Menstrual flow calculator works out Next period starts and Next period ends from First day of your last period, Average cycle length and Days of bleeding. The default figures on this page give next period starts of 2026-10-08; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
5 steps
- Using the stated 28-day cycle.
- Next period = 2026-09-10 + 28 days = 2026-10-08, running 5 day(s) to 2026-10-12.
- Ovulation is one luteal phase before that: 2026-10-08 - 14 = 2026-09-24, cycle day 15.
- The fertile window is the six days ending on ovulation: 2026-09-19 to 2026-09-24.
- On 2026-09-14 you are on cycle day 5, in the menstrual phase.
Predicted cyclestable
| cycle | periodStart | periodEnd | fertileWindowStart | ovulationDate |
|---|---|---|---|---|
| 1 | 2026-10-08 | 2026-10-12 | 2026-10-17 | 2026-10-22 |
| 2 | 2026-11-05 | 2026-11-09 | 2026-11-14 | 2026-11-19 |
| 3 | 2026-12-03 | 2026-12-07 | 2026-12-12 | 2026-12-17 |
Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
What the menstrual flow calculator calculates
Predicts upcoming period start and end dates, reports the current cycle day and phase, derives the fertile window, and assesses cycle regularity against the FIGO normal ranges.
How to use this menstrual flow calculator
Enter First day of your last period, Average cycle length, Days of bleeding and Cycles to predict. Next period starts updates as you type. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Next period starts, Next period ends and Next period starts on a.
The method follows Munro MG et al., FIGO classification system (PALM-COEIN) for abnormal uterine bleeding, System 1 normal limits, ACOG Committee Opinion No. 651: Menstruation in Girls and Adolescents - Using the Menstrual Cycle as a Vital Sign and Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle.
What you can compare
Change one input at a time to see how it affects Next period starts, Next period ends and Next period starts on a. The inputs you can vary are First day of your last period, Average cycle length, Days of bleeding, Cycles to predict and Today.
Understanding Next period starts, Next period ends and Next period starts on a
An estimate from calendar arithmetic, not a clinical assessment. Only about 4% of births occur on the estimated due date; ultrasound dating by a clinician supersedes menstrual dating when they disagree.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
What information do I need for the menstrual flow calculator?
You need First day of your last period, Average cycle length, Days of bleeding, Cycles to predict and Today. The page opens with default figures that give next period starts of 2026-10-08, so you can see the scale before entering your own.
What does this menstrual flow calculator show?
It shows Next period starts, Next period ends, Next period starts on a, Days until the next period, Cycle day today, Current phase, What is happening and Estimated ovulation, with the working and any warnings for your entries.
Can I check how the menstrual flow calculator result was calculated?
Yes. The “How this is worked out” section shows the calculation steps, and the sources section identifies the references used for the method.